fix(cron): flatten schedule schema for Gemini tool compatibility

[bohuslav-broza]

Summary

Gemini API (v1beta) currently does not support oneOf or const keywords in tool function declarations. Using z.discriminatedUnion in the cron tool's schema generates a JSON schema with these keywords, causing a 400 Bad Request (Invalid JSON payload) during initialization when using Gemini models.

Changes

• Refactored scheduleSchema in src/tools/cron/cron-tool.ts from a discriminated union to a flat object with optional fields.
• Updated descriptions for the now-optional fields (at, everyMs, expr) to indicate their requirement based on the selected kind.

Impact

• Fixes launch-blocking 400 errors for users with Gemini models.
• Maintains full functionality for existing scheduling logic (the cast to CronSchedule remains safe as the flat object is a superset of the possible union members).

Error Reference

⏺ Error: [Google API] [GoogleGenerativeAI Error]: Error fetching from https://generativelanguage.googleapis.com/v1beta/models/gemini-flash-latest:generateContent: [400 Bad Request] Invalid JSON payload received. Unknown name "const" at 'tools[0].function_declarations[10].parameters.properties[3].value.one_of...'